Advice for older trucks?

I’m not really new to diesels but looking for more opinions. I currently have a 2016 Titan XD with the 5.0, and despite their reputation, it’s been very reliable and I love it. That said, parts are getting really hard to find for it, so I’m considering selling it to buy a true 3/4 ton. $25k-30k is my max budget, but I want a nice truck. For me that means I’m looking at an LMM or maybe LML Duramax or an early 6.7 Cummins. Also would maybe consider a 6.0, but they’re not my favorite (would love a 6.7 Powerstroke though). All trucks I’m looking at are gonna have at least 150k on the clock, and I’m okay with that. What I really want is a truck I can build to ~400-500 hp and daily reliably for the rest of time without spending $150k to do so. I know the 68RFE is weak, so I’ll be looking for one with a built trans (with paperwork) if I go with a Cummins. What do you all think I should do?